Restore package os/signal, with new API:
Notify replaces Incoming, allowing clients
to ask for certain signals only.  Also, signals
go to everyone who asks, not just one client.

This could plausibly move into package os now
that there are no magic side effects as a result
of the import.

Update runtime for new API: move common Unix
signal handling code into signal_unix.c.
(It's so easy to do this now that we don't have
to edit Makefiles!)

Tested on darwin,linux 386,amd64.

Fixes #1266.

R=r, dsymonds, bradfitz, iant, borman
CC=golang-dev
https://golang.org/cl/3749041